The present invention relates generally to systems and methods for monitoring a subject's neurological condition. More specifically, the present invention is related to minimally invasive methods and systems for monitoring a subject who has epilepsy and determining if the subject is in a contra-ictal condition in which the subject is at low susceptibility for a seizure and is unlikely to transition into a pre-seizure condition within a computed or predetermined time period.
Epilepsy is a disorder of the brain characterized by chronic, recurring seizures. Seizures are a result of uncontrolled discharges of electrical activity in the brain. A seizure typically manifests itself as sudden, involuntary, disruptive, and often destructive sensory, motor, and cognitive phenomena. Seizures are frequently associated with physical harm to the body (e.g., tongue biting, limb breakage, and burns), a complete loss of consciousness, and incontinence. A typical seizure, for example, might begin as spontaneous shaking of an arm or leg and progress over seconds or minutes to rhythmic movement of the entire body, loss of consciousness, and voiding of urine or stool.
A single seizure most often does not cause significant morbidity or mortality, but severe or recurring seizures (epilepsy) results in major medical, social, and economic consequences. Epilepsy is most often diagnosed in children and young adults, making the long-term medical and societal burden severe for this population of subjects. People with uncontrolled epilepsy are often significantly limited in their ability to work in many industries. An uncommon, but potentially lethal form of seizure is called status epilepticus, in which a seizure continues for more than 30 minutes. This continuous seizure activity may lead to permanent brain damage, and can be lethal if untreated.
While the exact cause of epilepsy is often uncertain, epilepsy can result from head trauma (such as from a car accident or a fall), infection (such as meningitis), or from neoplastic, vascular, or developmental abnormalities of the brain. Most epilepsy, especially most forms that are resistant to treatment (i.e., refractory), are idiopathic or of unknown causes, and are generally presumed to be an inherited genetic disorder. Demographic studies have estimated the prevalence of epilepsy at approximately 1% of the population, or roughly 2.5 million individuals in the United States alone. Approximately 60% of these subjects have focal epilepsy where a defined point of onset can be identified in the brain and are therefore candidates for some form of a focal treatment approach.
If it is assumed that an “average” subject with focal epilepsy has between 3 and 4 seizures per month, in which each of the seizures last for several seconds or minutes, the cumulative time the subject would be seizing is only about one hour per year. The other 99.98% of the year, the epileptic subject is free from seizures. The debilitating aspect of epilepsy is the constant uncertainty of when the next seizure is going to strike. It is this constant state of uncertainty which causes epileptic subjects to remove themselves from society. It is the constant fear and uncertainty of when the next seizure will strike that prevents the person from performing activities that most non-epileptic subjects take for granted.
To that end, there have been a number of proposals from groups around the world for predicting seizures and warning the subject of the impending seizure. Most of such proposals attempt to analyze the subject's electroencephalogram or electrocorticograms (referred to collectively as “EEGs”), to differentiate between a “pre-ictal condition” (i.e., pre-seizure condition) and an “inter-ictal condition” (i.e., between seizures). To date, however, none of the proposed systems have proven to be effective in predicting seizures. Some researchers have proposed that seizures develop minutes to hours before the clinical onset of the seizure. These researchers therefore classify the pre-ictal condition as the beginning of the ictal or seizure event which begins with a cascade of events. Under this definition, a seizure is imminent and will occur if a pre-ictal condition is observed. Others believe that a pre-ictal condition represents a state which only has a high susceptibility for a seizure and does not always lead to a seizure, and that seizures occur either due to chance (e.g., noise) or via a triggering event during this high susceptibility time period. For clarity, the term “pro-ictal” is introduced here to represent a state or condition that represents a high susceptibility for seizure; in other words, a seizure can happen at any time. Ictal activity, within the scope of epilepsy, refers to seizure activity. Ictal activity may have other meanings in other contexts.